Well, in 1945, an A-bomb in Lenigrad and another in Moscow solves the Soviet problem, in fact it would have allowed the Americans and British to have truly shaped the world anyway they saw fit
I am a big fan of utopia so in my oppion there would not have been a Korean War, there would not be anymore wars becasue the the US and UK would control the world, but to be realistic humans would fine some way to mess everything up.
Without the bomb, the Soviets might have conquered Europe, they had 3 million soldiers and heavier tanks. In 1945 China was in civil war, but by Korea they had millions of soldiers, I think it would have been hard to win a conventional war espeacialy against both at the same time.
I think that even knowing how things played out it would still be a very hard call to make to go back in time and drop an A-bomb on Moscow.
I do agree with you that the Korean war would not have happened.
I am also a big fan of utopia but I don't think one could have been built out of the ashes. Not getting political but the UN split Korea to have a superpower assist in the aid and development of north and south. Even with good intentions bad things will happen.